In previous research on 12,630 individuals drawn from the Framingham Heart Study (FHS), we found evidence that one person's weight gain was associated with weight gain in others to whom he or she was connected. Here, we seek to extend that work by: (1) evaluating its causal basis in a new way, and (2) also examining smoking behavior. We will exploit known correlations of given variants of genes (alleles) and obesity and smoking, and to use allelic variation between individuals as an instrumental variable (IV). The key idea is that a person (an ego) may have peers (or alters) who are randomly assigned genes predisposing the alters to certain health behaviors, and that this random assignment can be seen as a kind of natural experiment, exposing the ego to peers who either exhibit or do not exhibit the pertinent behaviors. However, while the idea of using genes as instrumental variables in socioeconomic research is intriguing and has generated some excitement among social scientists, a careful review of the necessary assumptions for any such use is in order. Hence, we have three specific aims. First, we will embellish our FHS-Net dataset describing 5,124 individuals within a social network of 12,630 people, all of whom are measured repeatedly from 1971 to the present. Specifically, we will obtain and merge detailed genetic data to this file; the genes we will examine include at least the following: SLC6A3, CYP2A6, 5-HTTLPR, DRD2, INSIG2, TNFRS1B, LEPR, TNFA, and PPARG. Second, we will carefully evaluate several key assumptions underpinning the use of genes as IVs, especially in a network setting, such as the satisfaction of the necessary exclusion restriction and the stable unit treatment value assumption. We will also account for biological issues such as possible linkage disequilibrium in genes, and sociological issues such as the tendency to homophily. Third, using a two-stage IV estimation approach, we will assess whether obesity and smoking in an ego's alters (including friends, neighbors, spouses, siblings, and cousins) are causally related to similar behaviors in the ego. Our hypothesis is that obesity and smoking behaviors in alters will stimulate similar behaviors in egos and that this effect will vary according to the closeness of the relationship between the ego and the alter. We will examine these peer effects in the context of cardiovascular disease, which is responsible for 40% of deaths in the U.S. and incurs costs of over $350 billion annually. Our results could guide policy-makers by shedding light on the ways in which the embeddedness of individuals in social networks is relevant to their health. Our results could also open up new ways of exploiting